A catch of this kind is known from Swiss Pat. No. 618,049. In the case of this known catch, the snapped-in catch tongue is locked by the pivotably mounted releasing element. On the one hand, the releasing element can be pushed manually out of the locking position by means of the pushbutton. On the other hand, the releasing element can be pivoted out of the locking position by a releasing lever under spring tension. The releasing lever is normally fixed and can be released for automatic opening of the catch by means of a solenoid plunger when the latter is energized by an electric signal. The electric signal is generated in case of an accident-related impact by an inertia sensor. The electric signal generated by the inertia sensor is delayed electronically by a given period of about 8 seconds, for example, in order that, in case of an accident, the catch will open only after the accident is over. Since in an accident often several shocks occur in succession, the person must be sure to be held by the safety belt also during the following shocks. For this purpose, in the case of several impacts succeeding one another within the delay period, the delay period is started anew by the respective electric signal of the inertia sensor, so that also after the last impact the full delay period expires before the catch is automatically opened.
In the case of this known catch, the releasing lever must be brought back to its fixed position manually after each automatic release. If for instance for checking its functionality the catch is released automatically and when later closing the catch the user forgets to snap in the releasing lever manually, automatic release of the catch is not possible. The safety of operation of the catch is, therefore, not positively ensured.
The automatic release by means of the solenoid plunger and releasing lever is expensive to construct and requires a special design of the locking and of the opening mechanism of the catch. It is not possible to use the automatic release in connection with a time-tested and reliable design of a manually releasable catch.
From U.S. Pat. No. 3,311,188 a catch is known where for locking the catch tongue a wedge-shaped slider holds two relatively pivotable levers in engagement with the catch tongue. On the one hand, for manual opening of the catch, the slider can be moved by pushing by means of a pushbutton, and on the other hand, for automatic opening of the catch, it can be moved by pulling by means of a solenoid plunger acting on the opposite side. Through the movement of the slider, the latter releases the two levers, so that they are pivoted out of the locking position by means of an extension spring.
Also in this known catch the design of the opening mechanism is adapted specifically to automatic release. The automatic release, therefore, cannot be combined with a reliable, time-tested design of a manually releasable opening mechanism. The extension spring which pivots the locking levers out of the catch tongue must be very strong to be able to open the catch also when the catch is under a high tensile stress e.g. by the weight of the strapped person. The strong extension spring in turn causes strong friction between the slider and the locking levers, which must be overcome by the solenoid plunger. Therefore the solenoid plunger must be very large and hence expensive and bulky to ensure reliable automatic release.
From German Patent Disclosure DE-OS No. 23 63 973 an automatically releasing catch for safety belts is known. Here a bow or shackle connected with the safety belt is held by a plier. The plier is fitted to a nut which is displaceable by means of a threaded spindle driven by an electric motor. By displacing the nut, the plier is pushed against a stop and is opened by the latter. Opening of the catch occurs with a time delay relative to the accident-related shock that causes the starting of the motor, the delay period being determined by the duration of the displacement of the nut on the threaded spindle.
This known automatically opening catch is an independent structural unit which is not combined with a manually operable opening mechanism. The automatic releasing process is triggered in an accident by the first shock; resetting of the delay period in the case of several successive shocks is not possible. There is danger, therefore, that at the time of a later shock following the first shock the catch is already open. Lastly, after automatic release, e.g. for checking the functionality, the plier must be moved back into its closing position by manual turning of the threaded spindle.